Crop straw and N fertilizer applications impact paddy rice yield and greenhouse gas (GHG) emissions. However, their interactive effects have not been well documented. This study investigated the effects of straw (S), no straw incorporation (NS), and three levels of N fertilization rates (N0, N1, and N2) on single rice (SR), double rice (DR), and rice-wheat (RW) cropping systems. Straw incorporation significantly increased total CH₄ emissions by 118.6%, 8.0%, and 79.0% in the SR, DR, and RW, respectively, compared to the NS. The total GHG emissions in DR are significantly 72.6% and 83.5% higher than those in RW and SR, respectively. Compared to NS, straw incorporation significantly increased yield-scaled emissions by 27.8%, 15.0%, and 89.0% in SR, DR, and RW, respectively. Straw with N application significantly increased average rice yield over N1 and N2 by 39.4%, 50.0%, and 6.7% in SR, DR, and RW, respectively. There was a significant correlation between methyl coenzyme M reductase (mcrA) and CH₄ emissions in r_SR = 0.87 (p < 0.05) and r_RW = 0.85 (p < 0.05), except in r_DR = 0.06 (p > 0.05). This study scientifically supports straw incorporation combined with a moderate N application rate in rice-based cropping systems to maintain high rice yields and mitigate GHG emissions.

Straw returning has gradually been adopted as an effective approach to address the serious degradation of farmland. However, the carbon/nitrogen (C/N) ratio of rice straw is generally too high for microorganisms to decompose the organic materials and release nutrients, which may minimize the benefits of straw returning to the agricultural production system. This study aimed to investigate the effects of straw returning on rice production and propose optimum nitrogen (N) management for early rice production under a straw returning system. The total N fertilizer that was evaluated was 165 kg N ha-1, urea (46% N), applied in different proportions in three stages of rice cultivation: basal, tillering, and panicle. Using no straw returning with the N fertilizer ratio of basal:tillering:panicle = 5:2:3 treatment (T1) as the control, four different N fertilizer ratios of basal:tillering:panicle, including 5:2:3 (T2), 5:2:2 (T3), 5:4:1 (T4), and 5:5:0 (T5) were set under straw returning. The return of straw decreased the available N in the soil at the tillering stage, and impeded root growth and the crop canopy from establishing, which decreased the effective panicles by 10.1% compared with that of T1, limiting the increases in rice grain yield. Increasing the N fertilizer ratio 10–20% (T3 and T4) at the tillering stage effectively increased the content of soil ammonium and nitrate nitrogen, improved the root growth, and increased the root activities by 16.0–40.5% at the tillering stage. As a result, the effective panicle number increased by 5.1–16.2%. Among these, T4 treatment maximized the benefits of straw returning the most. Additionally, increasing the N fertilizer ratio at the tillering stage increased the shoot uptake across the early rice growing season and synchronized crop N uptake with the accumulation of carbon assimilates, which enhanced the crop growth rate and increased the rice yield by 13.5–25.1%. It is concluded that increasing the N fertilizer ratio by 20% at the tillering stage is a promising strategy to increase the availability of N in the phases of high demand for this nutrient.

Straw incorporation promotes rice productivity and soil fertility. However, the effects of tillage practice with straw on GHG emissions in paddy fields are not well documented. Under three rice-based cropping systems of China (single rice, double rice and rice-wheat cropping systems), we investigated rice yield, CH₄, N₂O, area and yield-scaled emissions arising from different straw-incorporated tillage patterns. Tillage with straw affected rice yield by −6.8~3.2%, −9.1~9.0% and −9.8~2.1% in single rice, rice-wheat and double rice cropping systems respectively. Straw impacted CH₄ emission but tillage influenced its impact irrespective of the rice cropping system. The highest CH₄ emissions occurred in RedT + S, RoT + S and RoT + S under single rice, rice-wheat and double rice cropping systems respectively. Cumulative CH₄ emission of PT + S decreased by 46.8% (p < 0.05) compared to RedT + S in the single cropping system, while under rice-wheat and double rice cropping systems, cumulative CH₄ emission of PT + S decreased by 19.0% (p < 0.05) and 13.2% (p > 0.05) respectively compared with RoT + S. Lower methanogenic abundance of PT + S translated into the lowest cumulative CH₄, area and yield scaled emissions in single rice and double rice cropping systems. To maintain high rice yield and reduce GHG emissions from straw incorporation, PT + S is recommended for a rice-based cropping system.

Co-incorporating rice straw biochar (RSB) and milk vetch (MV) is a promising strategy for sustainable rice production. However, the appropriate nitrogen (N) reduction rate under RSB + MV, particularly when balancing multiple production objectives, remains unclear. This study evaluated five N-reduction rates (0–70% conventional N fertilizer) under RSB + MV. Grain yield, quality, nutrient uptake, and partial factor productivity of N fertilizer (PFP_N) were assessed in early and late rice. Entropy-weighted Technique for Order Preference by Similarity to Ideal Solution (EW-TOPSIS) model was used to integrate all indicators, when Gaussian model was applied to estimate the appropriate N-reduction rate. Pearson correlation and random forest analysis identified primary predictors of grain quality. Results showed that appropriate N-reduction (≤30%) under RSB + MV maintained or increased grain yield, quality, and PFP_N compared to conventional N fertilization alone. Among all treatments, N₈₀BM and N₇₀BM achieved the highest comprehensive scores in EW-TOPSIS analysis, and Gaussian model-estimated N-reduction of 18.2% and 21.0% for early and late rice, respectively. Nutrient uptake, effective panicle number and filled grains per panicle were identified as primary predictors of grain quality. Therefore, under the tested condition, RSB + MV with 20% N-reduction achieved the highest overall performance. However, these findings require further validation across multiple-year and multi-location.

Although rice straw biochar (RSB) and milk vetch (MV) facilitate rice production and soil carbon and nitrogen (N) storage, their combined effects under N-reduction remain unclear, particularly at the aggregate-scale in reddish paddy soils. A two-year field experiment with four treatments: 100% N (N₁₀₀), 20% N-reduction + RSB (N₈₀B), 20% N-reduction + MV (N₈₀M), and 20% N-reduction + RSB + MV (N₈₀BM), was conducted in reddish paddy soil. Rice yield, soil organic carbon (SOC) and total N (TN) stocks, water-stable aggregate distribution and stability, and aggregate-associated C and N allocation were measured. Over the two-year study, N₈₀BM significantly increased grain yield by 11.2%, SOCs by 5.42%, and TNs by 6.92% compared with N₁₀₀ (p < 0.05). RSB and MV combined with N-reduction increased macroaggregate proportion by 7.92–11.8% and aggregate stability by 7.14–58.1%, while decreasing silt–clay fraction by 39.6–62.9%. Soil C and N enrichment occurred predominantly in macroaggregates. Pearson correlation and random forest analyses jointly identified aggregate distribution and stability as primary predictors of rice yield, SOCs, and TNs. Therefore, RSB + MV combined with 20% N-reduction sustained rice yield while enhancing soil aggregation and C, N sequestration, providing a rational strategy for N management in sustainable rice production.